The present invention relates to a method of and an arrangement for decoding and deciphering signals having been subjected on the one hand to an encoding of the MAC type and on the other hand to an enciphering by cyclic permutation around one or two points of cut.
The encoding of video signals according to the MAC (Multiplexed Analog Components) standard consists in ensuring for each of the lines of the picture the successive transmission of the analog chrominance and luminance components. The enciphering by cyclic permutation consists in cutting at the emission the picture signal at an arbitrary point and in permuting the two useful line fragments thus formed, the inverse cyclic permutation of course being effected upon reception in order to recover a clear representation of the picture signal thus scrambled. This enciphering occurs at a point of cut when one of the two chrominance or luminance components is subdivided into two signals or at two points of cut when each of the two components is subjected to the said cyclic permutation around a point of cut associated with it.
The problem which arises is that of sampling the different frequencies upon the emission and upon the reception of signals of the MAC type having been subjected to an enciphering upon emission. This problem can in fact be met if, for example, television signals should be emitted encoded according to a MAC standard with a picture format larger than the format 4/3 and/or with a resolution higher than that of the preceding emissions and these emissions should be received in a compatible manner by receivers of the first generation which are adapted to a picture format more strongly reduced (4/3) and/or to a lower resolution.
In order to give practical examples, the case can be imagined in which the emissions of the first generation correspond to the MAC standard defined in the publication SPB 284, 3.sup.rd revised version (December 1984) of the UER (Union Europeenne de Radiodifussion). Before the MAC encoding and enciphering, the signals are sampled at a rate of about 700 points for the luminance (sampling frequency 13.5 MHz) and of 350 points for the chrominance (sampling frequency 6.75 MHz). When these signals are encoded according to the MAC standard by utilizing a ratio r of the chrominance and luminance compression factors of ##EQU5## the sampling frequency f.sub.o at the output of the encoding/enciphering device is 20.25 MHz. The enciphering is obtained by cyclic permutation effected either on the luminance signal Y and the chrominance signal C separately or on the assembly of the signals Y and C, these signals being subjected to time shifts of a multiple of a base interval .tau..sub.o =1/f.sub.o .about.49 ns.
In order to increase the luminance resolution, it can be imagined to choose in the future a ratio r of the compression factors equal to 4 instead of 2. This is already possible when the signals are transmitted in amplitude modulation, for example, through cabled networks. This will also be possible in future for satellite transmissions in frequency modulation when the sensitivity of the receivers will have been improved and they can recover with a suitable signal-to-noise ratio, signals that have been subjected to time compressions by a factor 5 instead of the usual factor 3. With this ratio r=4, if the sampling frequency and the base interval used for the enciphering remain unchanged, the luminance signal comprises about 840 intervals instead of 700. In order to effect the deciphering from the same base interval as upon emission, in the receiver a luminance memory of 840 samples would thus have to be available instead of 700, which is not provided for in the receivers of the first generation constructed with memories of only 700 samples so that this solution becomes incompatible with these receivers.
In order to ensure that it is sufficient to use luminance memories of 700 samples, it is then possible to reduce the sampling frequency upon reception to the value f.sub.r =5/6 f.sub.o =16,875 MHz. It is then necessary to modify the addresses of points of cut (given by a pseudo-random address generator) by calculating the new addresses a'=5/6 a. Since upon emission, there are, for example, in the luminance signal 256 possible addresses separated by intervals equal to 2.tau..sub.o, the new address a' corresponds two in three times to a non-integral value. In the case of the chrominance signal, for which the addresses are separated by an interval equal to .tau..sub.o, five in six times the address a' is not integral. Since only a discrete number of samples (700 for Y, 175 for C) is memorized, the real address used in the integral value of a' closest to its theoretical value so that an error is made with respect to the temporary reference of each line which can reach half a sampling interval upon reception. A vertical line of the picture would consequently be recovered in fact as a slightly zigzag-shaped line, the peak-to-peak amplitude of this zigzag form attaining about the width of a sampling space, i.e. 1/700 of the picture width in the case of the chrominance and 1/175 in the case of the luminance.
When the frequency f.sub.r =5/6 f.sub.o, the division by 6 gives six possibilities of phase for the frequency f.sub.r. Consequently, there are six possible sampling signals: f.sub.ro, f.sub.r1, . . . f.sub.r5, which differ from each other by a phase distance equal to .pi./3. Upon passage from the signal f.sub.ri to the signal F.sub.ri+1, an advancement of .pi./3 of the phase of the sampling frequency occurs, which results in an advancement by a value equal to .tau..sub.r /6 of the sampling instants. During reading of the memory, this becomes manifest in the picture by a shift to the right of the corresponding line over a distance equal to 1/6.times.1/700 of the picture width.